1. Field of the Invention
The present invention relates to a shadow mask for forming electronic elements on a substrate and, more particularly, to a multiple shadow mask structure for use in a vacuum deposition process.
2. Description of Related Art
Active matrix backplanes are widely used in flat panel displays for routing signals to pixels of the display in order to produce viewable pictures. Presently, such active matrix backplanes for flat panel displays are formed via a photolithography manufacturing process, which has been driven in the market by the demand for higher and higher resolution displays which are not otherwise possible with other manufacturing processes. Photolithography is a pattern definition technique which uses electromagnetic radiation, such as ultraviolet (UV) radiation, to expose a layer of resist that is deposited on the surface of a substrate. Exemplary photolithography processing steps to produce an active matrix backplane include coat photoresist, pre-bake, soak, bake, align/expose, develop, rinse, bake, deposit layer, lift off photoresist, scrub/rinse and dry. As can be seen, the active matrix backplane fabrication process includes numerous deposition and etching steps in order to define appropriate patterns of the backplane.
A vapor deposition shadow mask process has been used for years in microelectronics manufacturing. The vapor deposition shadow mask process is significantly less costly and less complex than the photolithography process. It is, therefore, beneficial to develop ways of fabricating backplanes for large-area displays by use of the more cost-effective vapor deposition shadow mask process rather than by use of the costly photolithography process.
In a shadow mask vacuum deposition process, a layer of evaporant accumulates on the shadow mask with each deposition event and, thus, with multiple runs, multiple layers of evaporant accumulate. However, as layers of evaporant accumulate on the shadow mask, the shadow mask starts to deform, i.e., curl or warp, due to the accumulation of evaporant material on the shadow mask. More specifically, the deposited evaporant material typically develops tensile stress, largely due to shrinkage from cooling, which causes the shadow mask to become compressively stressed. The composite system of the shadow mask (with its surface compressed) with one or more layers of accumulated evaporant material (with its surface tensioned) will consequently bend or warp in order to equalize the total stress. This warping undesirably enables evaporant material to undercut the shadow mask, i.e., spread between the shadow mask and the substrate at the edges of one or more apertures, which results in irregularities in the pattern that is deposited and may even cause electrical shorts. As a result, the shadow mask must be changed or cleaned regularly, e.g., ≦10 deposition events, to avoid this problem. However, regular changing or cleaning is not practical in a continuous flow system because it is time consuming and costly. Moreover, cleaning tends to remove slight amounts of the mask material itself and, thus, cleaning may change the size of one or more apertures slightly. The problem is illustrated in more detail with reference to FIGS. 1, 2A and 2B.
FIG. 1 illustrates a top view of a conventional deposition mask 110, which is representative of a standard shadow mask that is suitable for use in a standard shadow mask vacuum deposition process. Conventional deposition mask 110 is formed of, without limitation, a sheet of nickel, chromium, steel or other metal. Formed within conventional deposition mask 110 is a pattern of apertures 112, which are openings of a predetermined size, shape and location, according to an associated circuit layout. During a standard shadow mask vacuum deposition process, evaporant material passes through apertures 112 for deposition upon a substrate (shown in FIGS. 2A and 2B), as is well-known. The overall dimension of conventional deposition mask 110 is user defined, and the thickness of deposition mask 110 is typically in the range of, for example, 20 to 40 microns, but may be from 10 to 100 micrometers.
Publications disclosing shadow masks and methods of forming and using shadow masks include U.S. Pat. Nos. 4,919,749; 5,139,610; 5,154,797; 6,156,217; 6,187,690; and 6,696,371, along with U.S. Patent Application Publication No. 2003/0193285.
FIG. 2A illustrates a cross-sectional view of conventional deposition mask 110, taken along line A-A of FIG. 1, in contact with a substrate 210 and prior to experiencing a deposition event. Substrate 210 is formed of, without limitation, anodized aluminum, flexible steel foil, glass or plastic. FIG. 2A shows that conventional deposition mask 110 includes a first surface 114 in intimate contact with substrate 210 and a second surface 116 which faces a deposition source (not shown) which supplies the evaporant material, such as, without limitation, metal, semiconductor, insulator or organic electroluminescent material, to be deposited via the evaporation process.
FIG. 2B illustrates a cross-sectional view of conventional deposition mask 110, taken along line A-A of FIG. 1, in contact with substrate 210 and after experiencing one or more deposition events which leaves a film or layer 212 of evaporated material on second surface 116 of conventional deposition mask 110. Second surface 116 of conventional deposition mask 110 becomes the “land area” for evaporant that does not pass through apertures 112 and, thus, layer 212 is formed thereon. Layer 212 is representative of evaporant material which has condensed and solidified on second surface 116 of conventional deposition mask 110 during one or more deposition events.
FIG. 2B illustrates the problem of conventional deposition mask 110 deforming between apertures 112 as a result of one or more deposition events. Specifically, between apertures 112, conventional deposition mask 110 curls or warps whereupon the edges of apertures 112 lift away from the surface of substrate 210. This curling or warping is caused by the difference in stress between conventional deposition mask 110 (with its surface compressed) and layer 212 (with its surface tensioned) that is deposited thereon.
As a result, the openings of apertures 112 become deformed and, thus, are no longer of the desired dimension. Furthermore, evaporant can spill into the resulting gaps between first surface 114 of conventional deposition mask 110 and substrate 210. This spillage is also known as “undercutting.” Consequently, there will be undesirable irregularities in the end product because the resulting structures deposited upon substrate 210 are not of the desired geometry or dimension.
What is, therefore, needed and not disclosed in the prior art is a method for increasing the number of deposition events that a shadow mask can tolerate without warping, thereby improving the efficiency and cost-effectiveness of a continuous flow shadow mask vacuum deposition process.